1. Field of the Invention
The present invention relates to a polishing apparatus, and more particularly to a polishing apparatus having a polishing section for polishing a workpiece such as a semiconductor wafer to a flat mirror finish and a cleaning section for cleaning the workpiece which has been polished.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnections is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 μm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. An abrasive cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the abrasive cloth and clamped between the top ring and the turntable. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the abrasive cloth is therefore polished to a flat mirror finish while the top ring and the turntable are rotating.
Further, in the polishing apparatus, abrasive slurry is supplied from a nozzle onto the abrasive cloth attached to the upper surface of the turntable. The abrasive slurry contains abrasive material such as silicon dioxide (SiO2) or cerium dioxide (CeO2) having a diameter of 1 μm or less in a liquid. The abrasive slurry contains water, abrasive material, and a small amount of dispersing agent to prevent aggregation of the abrasive material. Further, in order to perform chemical polishing in addition to mechanical polishing, acid or alkali may be added to the abrasive slurry.
The surface of the semiconductor wafer which has been polished is generally quite contaminated not only with particles of semiconductor material but also with abrasive material. The count of contaminants on the surface of the semiconductor wafer may be as high as 100,000 particles per wafer, and it is required to reduce this count to about 100 particles per wafer by some efficient method.
The conventional polishing apparatus could not be placed in a clean room because of dust particles generated by the polishing apparatus itself. Once the contaminants which adhere to the surface of the polished semiconductor wafer have been dried, it is difficult to remove the contaminants by cleaning. Therefore, the present practice is to preserve the semiconductor wafers which have been polished in water, immediately after polishing, in a specially constructed water-containing carrier which is brought into the clean room so that the semiconductor wafers may be cleaned in a cleaning device.
However, in the conventional apparatus, the cleaning device is spaced from the polishing apparatus and the semiconductor wafers must be transferred from the polishing apparatus to the cleaning device in such a state that they are preserved in water, thus productivity of the semiconductor wafers are lowered. Further, because the cleaning device itself becomes quite polluted from the dust particles adhering to the semiconductor wafers, it is not possible to use general cleaning devices provided in the clean room. That is, a special cleaning machine must be provided for exclusively cleaning the semiconductor wafers which have been polished. This leads to high apparatus cost.